Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a substrate or wafer. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. As design rules and process windows continue to shrink in size, inspection systems are required to capture a wider range of physical defects on wafer surfaces while maintaining high throughput.
One such inspection system is a scanning inspection system that illuminates and inspects a wafer surface. Light collected from the wafer surface is directed to a detector, or an array of detectors, for conversion to electrical signals useful for storage and analysis. Typical detector arrays are limited in their sensitivity due to significant detector noise. Often, this results in inspection systems that operate in a detector noise limited regime, rather than a photon limited, or surface limited regime. In some examples, detector noise is overcome by increasing the amount of illumination power. However, in high-power, laser-based inspection systems, increasing the power density of the incident laser beam may cause damage to the wafer surface. In addition, increasing the illumination power, particularly at short wavelengths, increases cost and may introduce reliability risks.
Previous inspection systems have relied on a variety of detectors, each with different advantages and disadvantages in specific applications. Exemplary detectors include photo-multiplier tubes (PMTs), charge-coupled devices (CCDs), PIN diodes, photodiodes, etc. Each of these detectors presents its own challenges and shortcomings. PMTs, for example, are typically bulky, and require high drive voltage. PMTs are also not available in large arrays. CCDs suffer from internal read-out noise mechanisms that limit their ultimate sensitivity compared with PMTs.
Avalanche photodiodes (APDs) are small sensors that provide significant gain and require lower drive voltage than PMTs. APDs may be configured in one of two operational modes. In linear mode, the voltage across the APD is set at a value below the break-down voltage. The output of the APD in this mode is a signal that is proportional to the amount of light detected. The gain of the APD may be set at relatively low values (e.g. 100×). In Geiger mode, the voltage across the APD is set at a value above the break-down voltage. In this mode, the gain of the APD becomes very large. Absorption of a single photon may give rise to a large pulse at the output that may be passed through a comparator to generate a clean TTL-like pulse. Thus, very high sensitivity may be achieved by APDs operating in Geiger mode.
However, once a Geiger pulse is trigged the APD is not responsive (i.e., “blind”) to the arrival of another photon until a period of time (i.e., the “quench time” associated with the APD) has passed. Once the APD pulse is “quenched”, the APD is again able to detect another photon. A typical quench time associated with an APD operating in Geiger mode is a few hundred picoseconds. Unfortunately, this period of blindness limits the dynamic range of APDs operating in Geiger mode, and thus limits their utility in current wafer inspection systems.
Improvements to the sensitivity and dynamic range of array based detectors employed in surface inspection systems are desired to detect defects on a wafer surface with greater sensitivity while avoiding thermal damage to the wafer surface.